Well I'm not sure it is understandable.
Everyone saw what happened to Armstrong. You can choose to go out that way or go 'yeah you got me'. I can at least give Lou a little bit of credit for fessing up and going to them rather than stoutly defend himself in the press. It just makes it worse for Cairns I think cause the guilt is so obvious. He'll eventually say, like Armstrong, that he is sorry about it all when the reality is that he is only sorry about getting caught.
I would be very intrigued to find out what he thinks he has that exonerates him in all of this. I personally would not call McCullum a liar as he has no need to make stuff up about Cairns. Why would he? What does he get out of it? In fact it looks worse for McCullum to wrongly finger Cairns considering he is an active cricketer and captain which is why its just so overwhelming.